The Toba eruption

Landsat photo of Sumatra surrounding Lake Toba

The Toba eruption (the Toba event) occurred at what is now Lake Toba about 67,500 to 75,500 years ago. The Toba eruption was the latest of a series of at least three caldera-forming eruptions which have occurred at the volcano, with earlier calderas having formed around 700,000 and 840,000 years ago. The last eruption had an estimated Volcanic Explosivity Index of 8 (described as "mega-colossal"), making it possibly the largest explosive volcanic eruption within the last 25 million years.

Bill Rose and Craig Chesner of Michigan Technological University have deduced that the total amount of erupted material was about 2,800 km³ (670 cu mi) — around 2,000 km³ (480 cu mi) of ignimbrite that flowed over the ground, and around 800 km³ (190 cu mi) that fell as ash, with the wind blowing most of it to the west. The pyroclastic flows of the eruption destroyed an area of 20,000 square kilometres (7,722 sq mi), with ash deposits as thick as 600 metres (1,969 ft) by the main vent.

To give an idea of its magnitude, consider that although the eruption took place in Indonesia, it deposited an ash layer approximately 15 cm (5.9 in) thick over the entire South Asia; at one site in central India, the Toba ash layer today is up to 6 m (20 ft) thick and parts of Malaysia were covered with 9 m (30 ft) of ashfall. In addition it has been variously calculated that 10,000 million metric tons of sulphuric acid or 6,000 million tons of sulphur dioxide were ejected into the atmosphere by the event, causing acid rain fallout.

The Toba caldera is the only supervolcano in existence that can be described as Yellowstone's "bigger" sister. With 2,800 km³ (670 cu mi) of ejecta, it was an even greater eruption than the supereruption (2,500 km³) of 2.1 million years ago that created the Island Park Caldera in Idaho, USA. The eruption was also about three times the size of the latest Yellowstone eruption of Lava Creek 630,000 years ago. For further comparison, the largest volcanic eruption in historic times, in 1815 at Mount Tambora (Indonesia), ejected the equivalent of around 100 km³ (24 cu mi) of dense rock and made 1816 the "Year Without a Summer" in the whole northern hemisphere, whilst the 1980 eruption of Mount St. Helens in Washington State ejected around 1.2 km³ (0.29 cu mi) of material. The largest known eruption since the Toba event, the Oruanui eruption in New Zealand around 24,500 BC, ejected the equivalent of 530 km³of magma.

The subsequent collapse formed a caldera that, after filling with water, created Lake Toba. The island in the center of the lake is formed by a resurgent dome.

Though the year may never be precisely determined, the season can: only the summer monsoon could have deposited Toba ashfall in the South China Sea, implying that the eruption took place sometime during the northern summer. The eruption lasted perhaps two weeks, but the ensuing "volcanic winter" resulted in a decrease in average global temperatures by 3 to 3.5 degrees Celsius for several years. Greenland ice cores record a pulse of starkly reduced levels of organic carbon sequestration. Very few plants or animals in southeast Asia would have survived, and it is possible that the eruption caused a planet-wide die-off.

There is some evidence, based on mitochondrial DNA, that the human race may have passed through a genetic bottleneck around this time, reducing genetic diversity below what would be expected from the age of the species. According to the Toba catastrophe theory proposed by Stanley H. Ambrose of the University of Illinois at Urbana-Champaign in 1998, human populations may have been reduced to only a few tens of thousands of individuals by the Toba eruption.